Glass reforming processes require a cold glass preform to be heated to a sufficiently low viscosity at which the glass can be reformed. As described in U.S. Publication No. 2010/0000259, published Jan. 7, 2010, a method for making a shaped glass article involves placing a flat glass sheet on a mold, heating the flat glass sheet (typically to a temperature at or near its glass transition temperature), and molding the flat glass sheet into a three-dimensional shape. The shaped glass sheet is removed from the mold and annealed. After annealing, the edge of the shaped glass sheet is contoured to a final precise dimension and finished. For a flat glass sheet, contouring of the edge of the glass sheet can be done at a low cost on a 3-axis machine. However, for a shaped glass sheet, if the edge contour is a complex curve, contouring the edge of the glass sheet to the final precise dimension may require a 4- or 5-axis machine, which is typically cost prohibitive. The previously mentioned U.S. Publication No. 2010/0000259 describes a second approach to reforming where the edge of a flat glass sheet is contoured and finished prior to the flat glass sheet being reformed into a shaped glass sheet. With this second approach, additional contouring and finishing of the edge of the shaped glass sheet is not needed.
In the second approach, the contoured flat glass sheet, which can have a complex shape, would have to be placed on the mold at a very precise position to form the shaped glass sheet with the final precise dimension. This precise positioning can be challenging because of thermal expansions and movement that occur during heating of the glass sheet and mold. Glass often has a very high thermal expansion when it is brought above glass transition temperature. For example, its expansion can easily exceed that of the mold material (where the mold material may be metal or ceramic). FIG. 1A shows a typical behavior for glass 100, made of Gorilla™ glass (available from Corning Incorporated, Corning, N. Y., under code 2317), and mold 102, made from Inconel 718. In FIG. 1A, the glass expansion exceeds that of the mold expansion above 600° C.
FIG. 1B shows placement of a flat glass sheet 1 on a horizontal mold 3 in a constrained condition. The flat glass sheet 1 is positioned on the top surface 2 of the mold 3 so that it covers the mold cavity 4. Alignment pins 5 are used to center the flat glass sheet 1 above the mold cavity 4 before the glass sheet 1 is heated and formed into the shaped glass article 6. If the flat glass sheet 1 is placed in the constrained condition shown in FIG. 1B before it is heated, the flat glass sheet 1 would simply break once it is near its glass transition temperature because it won't be able to expand. FIGS. 1C, 1D, and 1E show additional examples of constrained placement of the flat glass sheet 1. In FIGS. 1C and 1D, alignment pins 7 are used to locate the cold flat glass sheet 1 precisely on the mold 3 prior to heating and forming the glass sheet. In FIG. 1E, alignment blocks 8 are used for the same purpose as the alignment pins 7. The placements in FIGS. 1C-1E are over-constrained. To avoid breaking of the glass, one might consider heating the glass to a temperature at or near the glass transition temperature and then placing the heated glass on the mold in a constrained condition. However, this approach is not suitable for a thin glass (<2.5 mm) because the glass will warp and distort when heated and will no longer have the flat surface required for precise alignment on the mold.